Capacitive sensors are used extensively for level measurement and proximity detection. A conventional capacitive sensor, which includes one or more conductive plates, is sensitive to changes in the dielectric constants of materials or fluids near or surrounding the plates. The capacitive sensor detects the presence or lack of material in the vicinity of the plates by measuring the capacitance between the plates, which is proportional to the dielectric constant of the material filling the space between the plates. By measuring this capacitance, the quantity of material (for level measurement) or the existence of the material (for proximity detection) may be determined. Similarly, another conventional form of capacitive sensor, which uses linear electrodes, e.g., a long wire or strip immersed into a tank or storage bin holding a variable level of fluid or material, measures the level of the fluid or material by sensing and measuring the capacitance of the linear electrodes.
The accuracy of conventional capacitive sensors is based in large part on the dielectric constant of the material to be sensed. For example, when sensing capacitance, a fifty percent change in relative permittivity (the dielectric constant) causes a corresponding fifty percent change in the measured capacitance, i.e., the relationship is linear. Designing and producing capacitive sensors is therefore hampered by the sensitivity of conventional capacitive sensors to changes in the dielectric constant of the material to be sensed. One continuous capacitive level sensing system is the CLC series offered by SensorTechnics (www.sensortechnics.com) which appears to estimate a fill level of a container based on knowledge of the material being measured. If the material changes, however the sensor may require recalibration.
In U.S. Pat. No. 6,539,797 to Livingston et al, there appears to be disclosed a two electrode sensor embodiment wherein one is fully immersed and one partially immersed to measure material level independent of the dielectric constant of the material. It appears however that the measurements are processed in a more complex manner and calibration measurements are more numerous than the various embodiments disclosed herein.
There is a need for fill level sensing devices for containers that are independent of permittivity, easy to use, and require minimal to no calibration.